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Historical Changes of Eelgrass in Buzzards Bay
Sub-pages:
Codium Explosion off Wareham |
West Falmouth
Related pages: Physical Disturbance of Eelgrass | Eelgrass GIS Data
Historical Changes in Eelgrass Abundance
by Dr. Joe Costa
For the 2003 State of Buzzards Bay Conference, and in the Coalition for Buzzards Bay's 2007 State of the Bay report, the Buzzards Bay NEP estimated eelgrass cover from prior to European settlement in the watershed, to the present day. The analysis took into account the 1931-32 wasting disease that destroyed more than 99% of eelgrass beds in the North Atlantic, historical recovery of eelgrass in Buzzards Bay based trends at a few well studied sites during the 1950s, 1960s, and 1970s, and comprehensive assessments of eelgrass cover in buzzards Bay during the mid 1980s, 1996, and 2001. In the sections below are some of the assumptions included in the analysis.
Buttermilk Bay Eelgrass Cover in October 1951 versus September 1994.
Costa 1988 EPA Report in Thesis Data
With funding from the Buzzards Bay Program in 1985, Costa mapped current and historical distribution of eelgrass in Buzzards Bay based on field surveys between 1983 and 1987, historical aerial photographs between 1938 and 1983, and sediment cores (Costa 1988a, 1988b). A description of the methodologies used and the limitations of using older aerial surveys are found on the eelgrass GIS data page. Important considerations when evaluating old aerial photographs include whether or not the photographs were taken too early in the growing season to enable easy visualization of annual eelgrass beds, whether or not a recent hurricane or winter icing affected eelgrass abundance in shallow waters, and whether or not water transparency is adequate to enable visualization of the lower edge of the beds. With respect to the last issue, the ability to enhance aerial images in current GIS software (especially contrast and brightness) has proven especially useful in extracting information from the photographs.The 1988 eelgrass reports included historical changes of eelgrass abundance for selected sites. More recently, the Buzzards Bay NEP has been scanning and registering original source photographs and acquiring others, and using current GIS software and methods, creating new GIS coverages of eelgrass change that will be posted on this website.
1931-1932 wasting disease
A "wasting disease" destroyed virtually all eelgrass in Buzzards Bay (and elsewhere in North America) during 1931-32. The disease appears to have been caused by a virulent cellular slime mold that might have been introduced to the Atlantic or facilitated by exception warm weather during those summers. All documentation suggests that eelgrass populations equaled or exceeded present-day abundance prior to this catastrophe. Photographs taken 6 to 10 years after the disease show that eelgrass covered less than 10% of the present-day habitat area in Buzzards Bay, and many areas were not recolonized for decades. Most areas of Buzzards Bay had not fully recovered from the wasting disease until the mid 1960s.The process of recolonization was similar in many areas: new beds initially appeared on bare substrates, beds expanded, additional new beds appeared, and some beds were removed by disturbance. In this way eelgrass population saturated small areas (1-10 ha) 5 to 15 years after initial colonization. Rates of eelgrass colonization over larger regions (100's of ha) depended on distance from refuge populations and heterogeneities of the environment. The greatest rates of eelgrass expansion occurred during the 1950's and 1960's. Most available substrate was saturated by the 19801s, but eelgrass continued to expand in some areas, or show new pollution-related declines in others.
DEP 1996, 2001 and 1951 surveys
DEP first conducted aerial surveys of eelgrass in Buzzards Bay in 1996 (a recommendation in the Buzzards Bay 1991 Comprehensive Conservation and Management Plan). One problem with the aerial survey was that the photographs were taken June 1, before most functional annual eelgrass beds (that is, those growing from seed) were established. While DEP did conduct field survey during the following summer, such surveys only corroborate the presence or absence of eelgrass in a relatively few number of points. This made it somewhat difficult to compare some areas in the DEP survey, to the 1988 Buzzards Bay reports, which were largely based on a October 1981 coastal aerial survey where annual and perennial eelgrass beds show the greatest contrast with background sediments because of high leaf and epiphyte biomass. This 1981 aerial survey was also undertaken on a date and time with both a low tide and exceptional water clarity so that deep beds were also easily discerned. Because of these differences, it was difficult to determine whether the apparent absence of eelgrass in the 1996 survey was based on pollution, disease, or icing losses, or limitations of the aerial survey.It was not until, DEP repeated its eelgrass aerial and field survey in 2001 and 2002, and when we began reviewing other sets of aerial photographs that it became apparent that some eelgrass losses documented in 1996 in certain parts of Buzzards Bay (like in the Wareham River and off Wareham's Great Neck) were real and worsening (read this Codium versus Eelgrass off Great Neck article). The 2001 aerial survey was flown somewhat later (mid-June), but this time period still lead to underestimates of shallow annual bed cover. This survey also had only average water transparency, so some deeper beds were not mapped.
In 2003, DEP mapped eelgrass at a large number of sites in the Commonwealth using a 1951 state highway department aerial survey photograph set. These photographs were taken from a low altitude during the fall, and generally show good water transparency. Because many areas still had not recovered from the wasting disease, this eelgrass cover should not be used to determine the possible maximum of extent eelgrass in pollution and nitrogen TMDL studies, especially if aerial surveys from the 1960s, 70s, or 80s show a greater extent of eelgrass cover. DEP has not yet made the 1951 eelgrass survey coverage available through the MassGIS website.
Hypothetical Eelgrass Distribution During the Colonial Period
Estimates of eelgrass cover circa 1600 is a speculative exercise, but worthwhile in understanding the potential eelgrass distribution in Buzzards Bay without human impacts such as nitrogen loading, increased water turbidity associated with urban runoff, resuspension of sediments from boat traffic, and other human disturbances. Because eelgrass is largely dependent upon water transparency, and because areas exists in the such harbors in the Elizabeth Islands, and Nearby Martha's Vineyard and Cape Cod embayments that illustrate the depth that eelgrass may grow with relatively low nitrogen loading rates. That is, certain assumptions can be made about water quality based on existing eelgrass depth distribution on less developed nearby Islands, and assumptions about water quality and transparency. Based on these assumptions, historical eelgrass habitat in Buzzards Bay can be postulated. Such hypotheses can even be tested because Costa (1988, 1989) and others have shown that the remains of eelgrass seed coats (tests) are a valuable biostratigraphical marker that remain in the sediments of many bays and harbors which can be documented in sediment cores.For the estimation of eelgrass cover in 1600, the following two assumptions were made. First, eelgrass grew to 12 ft. MLW in upper Buzzards Bay, down to 20 feet MLW in lower Buzzards Bay, and second, half of the area between MLW and these boundaries were assumed to actually have eelgrass.
Hypothetical eelgrass cover circa 1600. From Costa 2003 State of Buzzards Bay presentation.
1980s vs. 1990s Changes in Distribution
Comparison between the Costa 1980s and DEP's 1996 mapped distribution of eelgrass.
Comparison between the Costa 1980s and DEP's 1996 showing decline in eelgrass cover in the Wareham area
Comparison between the Costa 1980s and DEP's 1996 showing decline in eelgrass cover in the West Falmouth Harbor area.
Comparison between the Costa 1980s and DEP's 1996 showing increase in eelgrass cover around Clarks Point New Bedford area.
Click to enlarge (200 kb jpg file).Comparison between the Costa 1980s, DEP's 1996, and the hypothetical precolonial distribution of eelgrass around Marion, MA.
Click to enlarge (1.8 MB jpg file).Comparison between the Costa 1980s, DEP's 1996, and the hypothetical precolonial distribution in all of upper Buzzards Bay.
Historical Summary presented at 2003 State of the Bay conference and new DEP data for 2001
Historical Summary of eelgrass in Buzzards Bay from Costa 2003 State of Buzzards Bay presentation and 2006 state of the bay report updates.
Eelgrass Links and Reports on Historical Changes
Costa, J. E. 1988a. Eelgrass in Buzzards Bay: Distribution, Production, and Historical Changes in Abundance. EPA 503/4/88-002 204 pp (5.4 MB)This report contains much the same information as the thesis work, except that it contains additional detailed embayment specific site histories.
Costa, J. E. 1988b. Distribution, production, and historical changes in abundance of eelgrass (Zostera marina L. ) in Southeastern MA. Ph. D. Thesis, Boston University, 395 pp.
(7.5 MB pdf)
DEP eelgrass interactive maps.
GIS shape files from MassGIS.
A May 12, 2004 letter to the Wareham Marine Resources Commission about the distribution of eelgrass in Onset Bay in response to a request for information.
PDF fact sheet on Massachusetts DEP eelgrass mapping program, and the Wetland Conservancy Program
MassGIS - DEP Eelgrass Data layer Description and download
NOAA Project: Loss of eelgrass at Cape Pogue Bay on Martha's Vineyard, Massachusetts, 1988-1994